1. Field of the Invention
The present invention relates to protection of a high voltage power supply circuit.
2. Related Background Art
Generally, since a high voltage power supply apparatus used in an electrophotographic process allows potential adsorption and jumping phenomena for a toner, a circuit obtained by serially connecting an AC high voltage generation circuit and a DC high voltage generation circuit as shown in FIG. 7 is widely used. In an ordinary monocolor process (charge, development, transfer and cleaning), its current value is relatively small. That is, the value is equal to or lower than 1 mA in a DC high voltage, while the value is equal to or lower than 10 mA in an AC high voltage.
FIG. 7 shows the structure of a general current protection circuit in the high voltage power supply apparatus.
In FIG. 7, a drive circuit 100 acts as a complementary circuit of an emitter follower system having such the structure as emitters of transistors 101a and 101b are connected to each other. Numeral 102 denotes a coupling capacitor which is used in combining AC current. Numeral 103 denotes a high voltage transformer, and numeral 106 denotes an input terminal. Numeral 108 denotes an output terminal from which a high output voltage Vo is generated.
Numeral 120 denotes a protection circuit against an excessive current. Numerals 121 and 122 denote reference voltage generation units composed of ladder resistors for generating a reference voltage used in setting of the output voltage Vo. Numerals 123 and 124 denote detection resistors which detect the output voltage Vo. Numeral 125 denotes a comparator which compares the output voltage Vo detected by the detection resistors 123 and 124 with the reference voltage.
Numeral 130 denotes a bias circuit which generates the DC high voltage, numeral 131 denotes a switching transistor, numeral 132 denotes a flyback transformer, numeral 133 denotes a high voltage rectification diode, numeral 134 denotes a smoothing capacitor, and numeral 135 denotes a current detection resistor.
Numeral 140 denotes a detection circuit which detects abnormality at a load 200, and numeral 141 denotes a coupling capacitor which passes only an AC component out of high voltage current waveforms detected by the current detection resistor 135. Numeral 142 denotes a rectification circuit which rectifies the detected AC voltage, numeral 143 denotes a comparator which compares the detected current waveform with the reference voltage, and numerals 144 and 145 denote reference voltage generation units each of which is composed of a ladder resistor for generating the reference voltage. An output terminal of the comparator 143 is connected to an input terminal of the comparator 125 through a diode 151.
Operations of the circuits connected as above will be described.
A high voltage current from the output terminal 108 flows into the ground through the load 200. Then, a DC high voltage component feeds back to the smoothing capacitor 134 through the detection resistor 135. On the other hand, the AC component feeds back to an end of the transformer 103 through the smoothing capacitor 134.
The DC current and the AC current are overlapped and flow in the detection resistor 135 arranged in a series of current channels including the load 200. Thus, for example, when a limiter control is performed by detecting the AC current, an AC voltage component in the voltage detected by the resistor 135 is guided into the coupling capacitor 141 used in combining the AC current, whereby only the AC voltage component is detected. The AC voltage is rectified by the rectification circuit 142, and then compared with a divided voltage being a current limiter start voltage defined by the resistors 144 and 145, by the comparator 143.
However, as the current value to be supplied from the high voltage power supply apparatus to the load, e.g., it is sometimes required to supply a larger current value instead of the value equal to or less than 10 mA.
Hereinafter, a concrete example will be explained.
In a color printer or the like, after performing a monochrome development process on a photosensitive drum, at least two colors are synthesized on an intermediate transfer medium called as an intermediate transfer body by using a multiple transfer method. Then, a transfer bias is applied on a printing paper sheet to transfer the synthesized color the sheet. After then, a thermal fixing process is performed by a fixing unit to discharge the sheet. On the other hand, even if a toner on the intermediate transfer body is transferred to the sheet, several percents of toner remains on the body. In case of performing a print sequence for plural sheets, since the remained toner is accumulated one after another. Furthermore, since the accumulated toner is superimposed on following images, stain portions are formed on the image, whereby image quality is deteriorated.
For this reason, in a color process, a cleaning sequence for the intermediate transfer body using a cleaning unit is an extremely important technique.
The intermediate transfer body is generally formed as a belt or a cylinder having a resistive layer. The toner on the photosensitive drum is adsorbed to the body by applying a voltage bias from a high voltage generation unit. The cleaning unit supplies an electrical charge to the toner according to the high voltage generated by a cleaning high voltage power supply unit. After the toner is adsorbed on the photosensitive drum, a cleaning process is performed by a cleaning blade provided on the photosensitive drum.
Since the cleaning high voltage power supply unit uniformly supplies the electrical charge to the toner, a high DC bias voltage and a high AC voltage are required.
The intermediate transfer body is constructed by the resistive layer and a high insulating layer on its surface. In a case where the body contacts with a charge roller to charge it, a relatively large electrostatic capacity is generated between the body and the roller. Although a value of the capacity depends on a printer size, such the value is generally several hundreds of picofarads to 1000 pF. On the other hand, a cleaning high voltage is determined from a printer throughput and a print density, and intensive corona is necessary to charge the toner. For example, a waveform of 2 kHz, 80% duty and 3 kV is required as the waveform of enabling to sufficiently show cleaning capability. As the waveform of an output pulse, a response speed equal to or faster than 50V/.mu.S is required. In order to apply such the output pulse waveform to the intermediate transfer body, a sufficiently lowered value is required for the output resistance of the high voltage power supply apparatus.
Almost every high voltage loads used in the electrophotographic process are electrostatic capacitors. Particularly, in the high voltage load or the like for cleaning, its flowing current of 90% or more is a current flowing in a dielectric load. On the other hand, in corona discharge and spark discharge between different electrodes and short-circuiting of the load, the flowing current of substantially 100% is an in-phase current flowing in a resistive component.
In a case where the load is a transfer body (i.e., charged member), electrically the load is an equivalent circuit of the capacitor. When the AC voltage is applied, a current corresponding to a known voltage change quantity flows. By such the dielectric current, a desired corona discharge is performed to the toner on the intermediate transfer body, and simultaneously a ground current is flowed through a basic layer (dielectric) of the transfer body itself. The flowing current is relatively large. That is, in case of an applied voltage having pulse waveform, the peak (i.e., peak value) of its current reaches several tens of milliamperes. In a case where an abnormal discharge current flows when a leak discharge is generated due to occurrence of an abnormality of the load such as transfer sheet winding around the intermediate transfer body, electrode short-circuiting or the like, it is frequently observed that such the abnormal discharge current has a relatively small value as compared with that of an ordinary load current of the transfer body.
For such the load abnormality, the above conventional high voltage power supply apparatus shown in FIG. 7 adopts a protection system of responding only to the excessive current by using the detection circuit 140, the protection circuit 120 and the like. However, in this protection system of responding only to the excessive current, it is impossible to practically protect the load. That is, although the high voltage power supply apparatus is protected, there sometimes occurs such a case as not detecting leak discharge (generally spark discharge) which is generated due to, e.g., the transfer sheet winding around the intermediate transfer body. This fact sometimes causes a damage to the apparatus.
As described above, the conventional high voltage power supply apparatus of low impedance has been realized. However, in the apparatus, spark energy at the leak discharge is high and a transitional current in the ordinary charge current reaches several tens of milliamperes. As a result, if the normal charge current is compared with the abnormal spark discharge current, there occurs such an inverted phenomenon as the absolute value of the normal charge current becomes larger than that of the spark discharge current. As above, there has been such a problem as the conventional apparatus for operating the protection circuit by observing only the detected current can not eliminate such the inverted phenomenon.